1. Field of the Invention
The present invention generally relates to a laser light source apparatus used in the field of optical communication, optical wavelength measurement, and the like, and particularly, to a tunable wavelength laser light source apparatus which can be used as a light wavelength (or frequency) control system, and which has a high mode stability, and a high wavelength setting resolution ability.
2. Description of the Related Art
With respect to conventional techniques of a tunable wavelength laser light source apparatus, the following three types of apparatuses are known.
(1) An external cavity laser light source apparatus PA1 (2) A semiconductor laser light source apparatus PA1 (3) A mutual optical injection-locking light source PA1 (1) The external cavity laser light source apparatus described as prior art has the following problem. PA1 (2) The semiconductor laser light source apparatus described as prior art further includes the following problem, in addition to the problem of the rotation mechanism of the diffraction grating and the problem of the setting wavelength resolution. PA1 (3) In the mutual optical injection-locking light source described as prior art, a synchronization range .DELTA..nu.0 decided by the injected light power amount occurs, as indicated by the equation (1).
As shown in FIG. 6A, in an external cavity laser light source apparatus, one end surface 21a of a semiconductor laser 21 is provided with an anti-reflection coating (AR coating). An optical cavity is constituted between a reflection mirror 22 (referred to as a diffraction grating 22 shown in FIG. 6A) provided on the AR coating surface 21a side with a collimating lens 20 interposed therebetween, and another high reflection (HR) surface 21b of the semiconductor laser 21, where no AR coating is provided.
Thus, laser oscillation is enabled by the gain of the semiconductor laser 21.
In this case, the reflection mirror 22 is constituted by a diffraction grating having a reflection wavelength selectivity and is additionally provided with a diffraction grating rotation mechanism 25 constituted by a piezo-electric element (e.g., PZT) 23 and a stepping motor 24, so that the resonating light wavelength is selected. Thus, the oscillation wavelength can be controlled over a wide band, by controlling the diffraction grating 22, as shown in FIG. 6B.
Here, in the diffraction grating 22, a number of grooves are formed on a substrate, at a concentration of several hundreds per mm. Incidence light which is directed at a predetermined incident angle to the diffraction grating 22 is diffracted at such an angle which makes light diffused by each groove satisfy Bragg diffraction condition.
Therefore, since the interval between the diffraction grooves is constant, the angle at which light is diffracted by the diffraction grating 22 changes if the wavelength of incidence light which is directed to the diffraction grating 22 changes.
Inversely, if the diffraction grating 22 is rotated and the incident angle of the incidence light is changed, the apparatus behaves in a manner in which the groove interval of the diffraction grating 22 substantially changes in accordance with changes in the incidence angle, and therefore, the wavelength of light diffused to a predetermined spatial position from the diffraction grating 22 changes.
In the case where this kind of diffraction grating 22 is used as a reflection mirror for an external cavity laser, the setting angle of the diffraction grating 22 is decided by a required wavelength and the number of grooves such that the diffracted light corresponds to the light axis of the incidence light. (This is called a Littrow arrangement.) In this Littrow arrangement, that wavelength which makes the reflection light so as to correspond to the optical axis of the incidence light is called a Littrow wavelength.
Therefore, spontaneous emission light of a wide wavelength band generated by an AR-coated semiconductor laser 21 is reflected and amplified by the diffraction grating 22, and therefore, the external cavity light source oscillates at the Littrow wavelength.
In this state, if the diffraction grating 22 is rotated, the Littrow wavelength changes and the oscillation wavelength of the light source can be swept.
This semiconductor laser light source apparatus is a light source technique using a C.sup.3 (Cleaved Coupled Cavity) laser constructed in a monolithic structure, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-274784. The C.sup.3 laser is a famous device known as a compound cavity laser.
As for the C.sup.3 laser, the structure and the principle thereof are taught in the reference "Applied Physics Letter 4(8), Apr. 15, 1983, pp. 650 to 652" and the likes, and the detailed description thereof will be omitted herefrom.
As shown in FIG. 7A, in this prior art technique, an AR coating surface 32a is formed by providing an AR coating on one of two semiconductor lasers 31 and 32 which constitute the C.sup.3 laser 30, whose active layers 35 are optically coupled with each other, and which are respectively supplied with injection currents If and Ir.
Further, a first external optical cavity is formed between a diffraction grating 34 and a creep surface 32b which are provided so as to the AR coating surface 32a, with a collimating lens 33 interposed therebetween.
As shown in FIG. 7B, in another structure, an AR coating is provided on the output end surface of the semiconductor laser 31, to form an AR coating surface 31a.
In practical use, the cavity lengths L1 and L2 of these first and second external cavities are made substantially correspond to each other.
According to this structure, the interval of a vernier mode in which first and second external cavities oscillate with the cavity modes of both external cavities corresponding to each other.
In the operation of the semiconductor laser apparatus, at first, mutual optical injection-locking is taken between the first external cavity constituted by the diffraction grating 34 and the creep surface 31b of one semiconductor laser 32 forming part of the C.sup.3 laser 30, and the second external cavity including the other semiconductor laser 31 forming part of the C.sup.3 laser 30.
In this state, the diffraction grating 34 is rotated by a drive mechanism 36, and thus, the oscillation wavelength as a laser light source can be changed over a wide band. In addition, injection currents If and Ir of two semiconductor lasers 31 and 32 constituting the C.sup.3 laser 30 are controlled by a control circuit not shown, and thus, the oscillation wavelength can be finely adjusted.
Further, in the case where the laser light source is constituted by the first and second external cavities, the external cavity lengths L1 and L2 of two external cavities are substantially equal to each other as has been described above, and therefore, the wavelength cycle in which an injection synchronization mode (or vernier mode) in which equal cavity modes are generated can be wide.
As a result of this, adjacent vernier modes can be set, beyond the limits of the wavelength resolution of the diffraction grating 34, so that the side mode suppression ratio (SMSR) can be maximized.
A mutual optical injection-locking light source as shown in FIG. 8 is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-106710, a first external cavity laser 44 is formed between a semiconductor laser (LD1) 41 in which one output end surface is provided with an AR coating to form an AR coating surface 41a, and a reflection mirror 43 provided on the AR coating surface 41a so as to oppose the AR coating surface 41a with a collimating lens 40 interposed therebetween.
In addition, a second, external cavity laser 45 is formed between a reflection surface 41b formed on the other end surface of the semiconductor laser 42 where no AR coating is provided, and a high reflection surface 42b formed on the other end surface of the semiconductor laser (LD2) 42 in which an AR coating surface 42a is formed on one end surface of the laser 42, with a pair of collimator lenses 40 are interposed between the reflection surfaces.
Further, this kind of mutual optical injection-locking light source utilizes a phenomenon that the light independently oscillated by the first external cavity laser 44 and the light independently oscillated by the second cavity laser 45 are mutually injected in their own cavity (shown in FIGS. 9A and 9B), thereby starting synchronized oscillation of a synchronized frequency.
The synchronized oscillation occurs when the difference in wavelength between the first and second external cavity lasers 44 and 45 oscillating independently from each other is within a synchronization range (or a locking range).
The synchronization range .DELTA..nu.0 of one single laser light source is expressed as follows. EQU .DELTA..nu.0={1/(4.pi..tau.p)}{Pin/P1}.sup.1/2 (1)
Here, .tau.p is a photon lifetime, Pin is an injected light power, and P1 is a light power inside a cavity.
Further, the synchronization range of the mutual optical injection-locking light source is expressed as a sum of the synchronization ranges .DELTA..nu.1 and .DELTA..nu.2 which are calculated independently from each other with respect to the first and second external cavity lasers 44 and 45.
In the state where the synchronization occurs, the oscillation gain close to the oscillation wavelength is suppressed. Therefore, if two cavity lengths are changed and the wavelength is swept with maintaining the state generating the mutual optical injection-locking (i.e., the state in which the vernier mode is generated) (as shown in FIG. 9C), it is possible to perform wavelength sweeping of a continuous phase.
In this case, even if the mechanical precision is low with respect to changes in cavity lengths, in correspondence of the mode wavelengths within a synchronization range or so is allowable, and therefore, assembly of a laser light source apparatus as a whole can be easily facilitated.
In addition, in this light source, since the injection light has an effect of increasing the Q value of oscillation, it is possible to obtain oscillation light having a high side mode suppression ratio.
However, the above three types of tunable wavelength laser light source apparatuses have the following problems.
Specifically, in order to obtain a wavelength variable range of a wide band (e.g., 100 nm), a rotation range of about 10.degree. is necessary as a rotation angle of the diffraction grating. However, a PZT element which can be electrically controlled ensures only a dynamic range insufficient for obtaining the rotation, and therefore, an actuator such as a stepping motor or the like must be used.
However, an actuator which generally has a large dynamic range cannot provide sufficient setting resolution and reproducibility, so that the light source using this actuator has a wavelength setting resolution limited to 0.1 nm.
In addition, the optical cavity modes establish every a free spectrum range (FSR) expressed as follows. EQU FSR=.lambda..sup.2 /2nl (2)
Here, .lambda. is a light wavelength, n is a refraction index, and l is a cavity length.
Therefore, if the external cavity is constituted with use of a diffraction grating, the cavity length l is long, so that the modes establish with a high density.
Meanwhile, the wavelength resolution of diffraction grating .DELTA..lambda. is expressed by the following equation. EQU .lambda./.DELTA..lambda.=N.times.W (3)
Here, .lambda. is a wavelength of an incidence light, N is the number of grooves per 1 mm, and W is the length of an irradiation area on the diffraction grating surface relating to diffraction.
For example, where the cavity length is 40 mm, the wavelength is 1.55 .mu.m, and the groove pitch of the diffraction grating is 1100 line/mm, and the light spot diameter is 2.5 mm, the FSR and the resolution of the diffraction grating are respectively 30 pm and 270 pm from calculations.
If this relationship is applied to FIG. 6B, the wavelength resolution of the diffraction grating is decided by the grating discrimination curve, and includes several modes of the external cavity.
Therefore, the diffraction grating cannot dissolve one cavity mode, and an apparent difference in reflection ratio does not appear between an oscillation mode and a mode adjacent thereto, so that the side mode suppression ratio cannot be set to be high.
In addition, it is not possible to control which of the modes existing within the range of the resolution causes oscillation. Consequently, an instability of oscillation mode occurs.
This instability of oscillation mode is a factor which causes degradation in setting wavelength resolution and reproducibility of the oscillation wavelength of the entire light source.
Further, the AR coating provided on an end surface of the semiconductor laser generally has a reflectivity of about 10.sup.-3. However, in order to eliminate influences from the cavity mode (or internal mode) inside the semiconductor laser, a reflectivity of 10.sup.-4 or less is required.
Therefore, in the case where a light source is prepared by providing a conventional AR coating on an end surface of the semiconductor laser, the wavelength jumps between internal modes, therefore the light source has a wavelength area where oscillation is impossible.
Specifically, this light source apparatus is based on a 4-mirror system, phase mismatching occurs at a cyclic wavelength position, due to an air gap between semiconductor end surfaces formed by creeping, so that a jump of an oscillation wavelength is caused like an instability of the oscillation mode and incompleteness of the AR coating.
Further, in this light source apparatus, a fine adjustment is performed on the oscillation wavelength, with use of an injection current. However, since this light source apparatus has an external cavity structure, the optical cavity length is long so that changes in optical distances caused by injection currents are very small in relation to the optical cavity length. Therefore, there is a problem in that the fine adjustment range of the wavelength is small.
This value greatly changes, depending on the phase conditions of light to be coupled.
This change is expressed as follows. EQU .DELTA..nu.={.DELTA..nu.0(M.sup.2 +2M cos .beta.+1).sup.1/2 }/(M+1)(4)
where .DELTA..nu.1 and .DELTA..nu.2 are synchronization ranges independently generated by two external cavity lasers, M is (.DELTA..nu.1/.DELTA..nu.2), .DELTA..nu.0 is (.DELTA..nu.1+.DELTA..nu.2), and .beta. is a relative phase difference of coupled light.
Therefore, the synchronization range, is wide, and therefore, it is possible to perform sweeping even if the mechanical precision is low with respect to the mode sweeping of the cavity. However, there is a problem in that a mechanical swing causes a swing of the synchronization range, so that the synchronization state is unstable.